1. Technical Field
The present invention generally relates to a method for producing polymer foam by extrusion. More particularly, the present invention relates to a method for producing low-density, high melting point polymer foam by extrusion.
2. Background Information
For some time, low-density polystyrene foam (also referred to as xe2x80x9catacticxe2x80x9d polystyrene foam) has been found useful in insulation, packaging, beverage cups and food containers. However, polystyrene foam generally has a service temperature limit of about 366.30xc2x0 K. Above the service temperature limit, items made from the foam will warp and distort. Therefore, there is a general desire for other types of low-density foam that do not suffer such shortcomings.
Polymer resins, such as, for example, poly(ethylene terephthalate) (often referred to as xe2x80x9cPETxe2x80x9d) and syndiotactic polystyrene, exist that could be used without such shortcomings. PET is currently widely used to make many recyclable plastic items, such as soda bottles. However, attempts to produce low-density foam from polymers with xe2x80x9chighxe2x80x9d melting points (i.e., greater than 505xc2x0 K) has proven to be difficult, and the quality of such foam has been poor. Polymer foams extruded using single traditional blowing agents, such as, for example, carbon dioxide, chloro-difloro-methane and butane, have experienced foam cell collapse and/or severe corrugation. Thus, the quality of the foams produced has not been close to polystyrene. The problem of collapse is due to the high foaming temperature such materials demand, for example, PET foams at about 516xc2x0 K. Using conventional blowing agents at such temperatures results in a high rate of expansion, causing cell wall rupture and allowing the gas to escape. Without gas in the foam cells prior to cooling, the cells cannot support themselves. In addition, many such polymer resins are crystalline in nature, and as such, have an inferior melt strength compared to polystyrene resins. As one skilled in the art will know, melt strength refers to the ability of a material to be stretched at its melting temperature without breaking. The combination of a lower melt strength and higher vapor pressure at the foaming temperature also requires a reduction in the size of the extrusion die opening where the foam exits. Such small die openings lead to a thin gauge foam sheet experiencing severe corrugation at low densities.
Thus, a need exists for a way to make a quality low-density, high service temperature polymer foam approaching or achieving the quality of polystyrene foam.
Briefly, the present invention satisfies the need for a quality low-density, high service temperature polymer foam by combining a high boiling point blowing agent providing plastization and volume with a low boiling point blowing agent providing the vapor pressure needed to prevent foam cell collapse during cooling.
In accordance with the above, it is an object of the present invention to provide a method for producing polymer foam by extrusion.
It is another object of the present invention to provide a method for producing low-density polymer foam.
It is still another object of the present invention to provide a method for producing low-density, high service temperature, substantially uniform closed cell polymer foam.
It is yet another object of the present invention to provide a method for producing low-density, high service temperature, substantially uniform closed cell polymer foam with a surface that is substantially smooth to the touch.
The present invention provides a method for producing a substantially uniform closed cell foam of density less than 0.25 g/cm3 from a crystalline polymer resin by extrusion through a die at a foaming temperature. The method comprises steps of heating the crystalline polymer resin to a foaming temperature (for example, above 543xc2x0 K for polyester) to melt it, selecting a blowing agent combination, combining the blowing agent combination with the resin to create a mixture, cooling the mixture to a temperature approaching a freezing temperature for the mixture (but not below the freezing temperature), and extruding the foam from the die. The blowing agent combination comprises 50 to less than 100 mole percent of a first blowing agent having a boiling temperature at STP of greater than 310xc2x0 K, and more than 0 to 50 mole percent of a second blowing agent having a boiling temperature at STP of less than 310xc2x0 K. The blowing agent combination has an equilibrium solubility pressure in the resin of less than about 45 atm at the foaming temperature, and greater than or equal to 1 atm at a glass transition temperature.
These, and other objects, features and advantages of this invention will become apparent from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings.